A
philosophical zombie is a being physically indistinguishable from an
actual or possible human being, inhabiting a possible world where the
physical laws are identical to the laws of the actual world,
but which completely lacks consciousness. For zombies, all is dark
within, and hence they are, at the most fundamental level, utterly
different from us. But, given their definition, this singular fact
has no direct implications about the kind of motion, or other
physical processes, the zombie will undergo within its own world.
Under quite standard physicalist
assumptions, such as certain assumptions about the 'initial
conditions' of the zombie's world and that of the causal
closure of the physical1,
a zombie's behaviour, as well as its
underlying physical state, should be indistinguishable from the
behaviour and physical state of a genuine human being.

The
first case envisaged above is that of the beloved 'zombie
duplicate' -- in particular, my philosophical
zombie is one which is physically indistinguishable from me. This is
the case most usually invoked in discussion, since it can be granted
that I, at least, definitely do possess consciousness. But if we
grant that human beings in general are conscious beings then the
second case will serve our philosophical thought-experimental
purposes just as well, while avoiding the (perhaps ultimately
irrelevant) complications that perfect physical duplication might
involve. For example, it is arguable that the only possible world
that could have a perfect
physical duplicate of me would have to be totally physically
identical to this world and hence might be the very same possible
world. This problem could arise independently of any concerns we
might have that consciousness is somehow a relational property, via
an assumption of the complete causal inter-connectedness of the
physical realm.

It is, of course, far from clear that
any two worlds that are physically indistinguishable are thereby the
same world, but assuming otherwise at this stage seems to come close
to begging one side of the question to be explored here. However, I
don't think this worry is really very plausible, even if we
accept physicalism, since it appears quite
possible for there to be physical things which are completely
causally isolated from each other. Many modern cosmological models
allow for this, or even demand it. If so, possible worlds could
differ physically in ways that have no effect on the physical states
of certain parts of them, and hence there could well be a physically
different world with a physically indistinguishable duplicate of me
in it.

Before considering the mere logical
possibility of philosophical zombies, I want to digress briefly on
the matter of their actuality or nomological possibility. A
real zombie would be an actual being who is either physically
identical to some human being, or is physically identical to some
genuinely possible human being but who is utterly lacking in
consciousness. To assert that zombies are nomologically possible
would be to assert that in some world that shares all of its laws
with the actual world there is a being identical to some actual or
genuinely possible human being who is utterly lacking in
consciousness. Of course, the existence of a real zombie would entail
that zombies are nomologically as well as logically possible, but the
reverse entailments do not hold.

The robust sense of reality so
necessarily lacking when discussing the logical possibility of
zombies, should instantly reassert itself if we ask, even while, for
the moment, granting the possibility of zombies, whether there are,
or are likely to be, or ever have been, any real or even just
nomologically possible zombies. Clearly, the question of whether
there are any philosophical zombies actually lurking among us is a
form of the venerable problem of other minds. I take it that this
question deserves the same kind of answer as other distinctively
philosophically skeptical questions, such as whether the world might
have been created five minutes ago, or whether there is any
'external' world at all.

Note an important difference here
between skeptical questions like those of other minds and external
reality and what I take to be quite non-skeptical, though
distinctively philosophical, worries, such as the problem of the
freedom of the will. The problem of freedom depends upon a tension
between commonsense, intuition and certain interpretations of what we
know (or think we know) about the laws of nature. This tension is
such that the intuitive appeal of belief in the existence of free
will seems to be in prima facie conflict with scientific
knowledge, and the familiar arguments against the existence of
freedom exploit this tension in various ways. The skeptical
hypotheses are not like this. They conflict equally with all of
intuition, commonsense and what we know about the laws of nature.
Thus trying to defend seriously either the actual existence or the
nomological possibility of zombies would require denying those laws
of nature which seem to link physical states to states of
consciousness. Of course, we don't know very much about these
laws, but it is already abundantly clear that there are any number of
quite particular regularities between neural systems, states and
processes and varieties of conscious experience, and many of these
are already being scouted out by our rapidly developing neurosciences
(for just one striking example see Tong et.al. 1998).

So the worry that there are real
philosophical zombies somewhere hereabouts, or that they are even
nomologically possible is a kind of skeptical worry. How, in general,
should we respond to specifically skeptical challenges? It would be
great if we could show that the challenge was incoherent, but that's
a rare treat. More typically, it seems evident that the skeptical
worry is a worry at all just because it is prima facie
coherent, that is, it seems to be at least logically possible. For
example, I don't think there is much doubt that the hypothesis
that, for example, the universe was created five minutes ago in
precisely the state it was in five minutes ago is coherent, and that
is tantamount to saying that it is logically possible that the world
was created five minutes ago. Similarly, it seems fairly obvious that
it is logically possible that I am in the matrix right now, or being
deceived by the evil genius about the very existence of an external
physical world.

But I don't believe that any of
these hypotheses are true; in fact I regard them as spectacularly
unlikely, and in violation of certain physical laws which I take to
hold in the actual world. You probably agree with me on this.
Furthermore, I think that I know that the world was not
created five minutes ago, despite the fact that I concede that a
'radically young' universe is logically possible.
Philosophically speaking, I think that any epistemological theory
which led to the conclusion that I did not know that the world was
very old solely on the basis of the logical possibility of the
opposite and utterly independent of whether or not it is true that
the world is very old, would be a deeply flawed account of knowledge
(that's not to say such flawed account have never been
offered).

Roughly speaking -- though
philosophy has shown there are many subtle niceties to the subject --
a proper epistemology should endorse a kind of 'conservatism of
belief', which can be pugently expressed via the mechanic's
cliché 'if it ain't broke, don't fix it'.
My current belief that the universe is quite ancient serves me well.
It fits in with many other of my beliefs. It does not lead me astray
and is amazingly well able to integrate with new evidence that I pick
up every day talking with others (say about what happened yesterday),
reading the newspaper or browsing the web science pages. This belief
has passed all the epistemic tests it needs to count as knowledge2
(though it may begin to fail these tests at any time). One might
complain that just fitting in with a general system of belief and
evidence cannot be enough for knowledge. Quite so. What is needed in
addition is simply the truth of the belief. But, since the world is
in fact very old, I do know that fact.

We are in exactly the same sort of
epistemic position with regard to the question of real or even just
nomologically possible zombies. I don't believe there are any.
Nobody can give me, nor is there any, evidence that it is zombies
that surround me rather than fully conscious human beings. So my
belief is well supported, stable and unassailable. I am under no
epistemic pressure to change or even examine this belief and can
remain secure in my knowledge that there are no philosophical zombies
and that in fact the laws of nature which link brain states to states
of consciousness rule out zombies as nomologically impossible.

But
this knowledge does not show that philosophical zombies are logically
impossible. It is much
harder to show any such a thing. Nonetheless, some
philosophers contend that philosophical zombies are logically
impossible, and this paper will focus on one argument recently
advanced by Robert Kirk (1999). It is claimed that there simply are
none of the possible worlds invoked in the zombie definition. No
being physically identical to me, in the appropriate sense, could
lack consciousness3.
This is a strong claim. It is far stronger, for example, than the
claim that zombies are nomologically
impossible, for this only asserts that in any possible world that
shares all its 'natural laws' with our world any physical
duplicate of me will be as conscious as I am. The claim that zombies
are nomologically impossible is also distinct from the claim that
they are physically impossible. Distinguishing nomological
and physical possibility assumes that the realm of natural law might
(logically might) outrun that of mere physical law. Notice that if in
fact all natural laws are physical laws or logically supervene upon
physical laws then there are no worlds that agree on our physical
laws but differ in some natural law. Then, since there are evidently
laws which link physical states and states of consciousness, the
logical possibility and the nomological possibility of zombies would
come to the same thing, for then any world that differed in its
natural laws from our world would also differ in some physical law.
But, as we shall see, there is very little reason to collapse this
distinction4.

On
the other hand, if the laws which link physical with non-physical
properties are 'natural' without being, or logically
supervening upon, physical laws then zombies could arise by breaking
these natural laws without breaking the physical laws. If mental
properties are non-physical, then it seems quite reasonable to claim
that laws linking the mental and physical are themselves not physical
laws (in just the way it would be reasonable to deny that laws of
economics are physical laws). The issue would then come down to
whether or not such natural, but non-physical laws, logically
supervene upon the physical laws. But, obviously, simply to assume
that all natural laws logically supervene upon physical laws would
beg the question against the logical possibility of philosophical
zombies. Or, in other words, it would suffice to show that zombies
are logically impossible to show that all natural laws logically
supervene upon the actual physical laws5.
But that looks to be extremely hard to show. If any kind of
'non-physical world' is logically possible and if it is
logically possible for such a realm to enjoy its own set of natural
laws then the supervenience claim would
obviously be in jeopardy. I have no idea how one could even begin to
argue that such lawful but non-physical realms are logically
impossible, and am equally at a loss see how the laws of such realms,
if their possibility is granted, would have to be logically
supervenient upon the laws of our physical realm. For one thing,
since such realms would agree on all their physical laws (trivially)
the supervenience claim entails that they would have to agree on all
their laws -- so at most one
such realm would be possible. There would still be many different
possible non-physical worlds since they could presumably differ in
their 'initial conditions'. But they would have to share
their laws. One might claim that such realms could differ in their
'substance' while agreeing in their 'laws'
but I really have no idea what such a claim really means. Nor do I
see how 'mixed realms' that contained both physical and
non-physical components would, of logical necessity, have any and all
of the laws which govern the non-physical side of things depend upon
the physical laws. So this seems to be a very ambitious and
hence not a very promising way to attack the idea of philosophical
zombies.

And, contrary to the opinions, or at
least the hopes of many, the idea of zombies is important. For to
claim that zombies are logically possible is to deny a very common
form of physicalism. I want to emphasize this point: it is the mere
logical possibility of zombies that refutes physicalism. If, say, my
philosophical zombie is logically possible then there is a possible
being which shares all my physical properties but does not share all
my mental properties. Thus mental properties are non-physical
properties and the physicalist assertion that everything is
ultimately physical is false. An ontologically liberal functionalist
cannot escape either, since the idea of a 'functional zombie'
is a simple extension of the idea of a 'physical zombie'.
In fact, if, as seems reasonable, we assume that functional
properties logically supervene upon physical properties, then the
possibility of a philosophical zombie refutes functionalist
physicalism no less than it refutes 'bare physicalism'
(see Chalmers 1996, especially chs. 3 and 4).

This is a real problem. I've
sometimes heard it said that the idea of zombies -- like that of
certain other bizarre, purely philosophical thought experiments --
is so weird that we just don't know what to say about them, or
that we just have no way to assess their logical possibility. And
therefore (therefore?) we needn't spend any time worrying about
them. Not good. This is tantamount to saying that we don't know
what to say about the truth of physicalism and have no way to assess
its truth or falsehood. Good friends of physicalism ought not to take
this line. They know perfectly well what to say about zombies: such
monstrosities are not logically possible. The question is
whether there is any way to convince someone who is neutral about
physicalism of this without begging the question.

I've also heard it said that
physicalism is not meant to be such a 'strongly metaphysical'
claim that it would have such exotic implications about the
nethermost regions of logical space; rather, physicalism is supposed
to be a kind of quasi-scientific, empirical claim that the creatures
of this world, and in particular human beings, are purely physical
creatures. And, just as physics doesn't care that it is (or
might be, so to speak) logically possible for quarks to have
radically different properties in some merely logically possible
worlds, philosophers shouldn't care about hypothetical
possibilities of Cartesian minds, or whatever other wild psychical or
ectoplasmic metaphysics one might dream up. This sort of reply misses
the point, and the strength of the zombie challenge.

Consider that the mere denial of
physicalism does not entail that zombies are possible. The former is
entailed by the existence of Cartesian possible worlds -- worlds
in which there are non-physical entities and properties: mental
substances possessing mental properties as postulated by Descartes.
But there could be Cartesian worlds even if philosophical zombies are
logically impossible6.
So it is not at all the case that the zombie hypothesis is just a
fancy way of dressing up anti-physicalism.

At the same time, I would think that
philosophers are right to be wary of asserting that physicalism is
logically necessary: that there are absolutely no possible worlds
that contain non-physical entities and/or non-physical properties.
The metaphysical visions of Leibniz or
Spinoza, for example, do not appear to be
flat out impossible.

Such modesty does not weaken the zombie
argument, in fact it is irrelevant to it. For the possibility of
zombies just shows that consciousness is not itself a physical
property, nor even a property which logically supervenes upon such.

Of course, we don't have a very
clear idea of exactly what makes a property a physical versus
a non-physical property. But we can use the rationale of the
zombie argument itself to help us get clearer about such properties.
Here is a first pass at a sufficient condition for being what I will
call a radically non-physical property:

(R) if some thing, x, has a property, P,
which is such that it is logically possible for something physically
indistinguishable from x to lack that property then P is a radically
non-physical property7.

My
philosophical zombie, if it is logically possible, will reveal that
there are mental properties of consciousness which are radically
non-physical properties by this criterion. And that seems intuitively
correct. However, some properties which intuitively present no
difficulties for a physicalist outlook are hereby declared radically
non-physical. Any relational property will come out as non-physical
by this criterion. For example, a Canadian one dollar coin has a
possible physical duplicate which is not money at all. However, it is
easy to extend our criterion of non-physicality to cover relational
properties thus:

(R*) if some thing, x, has a property,
P, which is such that there is a possible world which is physically
identical to the actual world but in which x's physical
duplicate lacks P, then P is a radically non-physical property.

Note
that the specification of the possible world that tests for
non-physicality might have to include the total history of the world.
If the actual world was in fact created five minutes ago then it may
be that there is no real money in it (only lots of perfect
counterfeits), if one defines money in terms of being printed by a
legitimately authorized mint (as opposed to miraculously appearing
out of nowhere with the rest of the world and being treated as money
by the similarly newly created denizens of that world). Thus test
worlds may have to be totally physically identical to the actual
world. This identity might even have to extend through 'future
history' as well. For example, it is conceivable that the
property of inertia is a relational (and physical of course) property
depending upon the total distribution of
matter throughout all of space and time. So if -- as
seems unlikely to me -- properties of consciousness are in fact
relational properties then it will be the logical possibility of
whole 'zombie worlds' that will reveal that such
properties are radically non-physical.8

We might now define a rather weak form
of physicalism simply as:

(P) There are no radically non-physical
properties.

Now
it is clear that the mere logical possibility of zombies is enough to
refute physicalism insofar as it reveals the radically non-physical
nature of at least some mental properties and insofar as any
reasonable version of physicalism (such as (P) for example) rejects
the existence of such radically non-physical properties.

Showing that philosophical zombies are
logically impossible would thus make the world safer for physicalism.
But on the other hand, a proof of the impossibility of zombies which
rested upon a premise asserting or implying the truth of physicalism
would be worthless. That would just beg the question.

Robert Kirk has recently (1999)
presented an argument attempting to demonstrate that philosophical
zombies are logically impossible. I think the argument does not
succeed, but its failure is instructive, although in the end not
surprising. Kirk proceeds by reductio, assuming that a zombie
is possible and from that deriving a contradiction. His method
involves supposing that a zombie suddenly acquires qualia. To
prevent misunderstandings based on this notoriously troublesome term,
let me reassure the reader that no pernicious
assumptions about the nature of qualia are at play here: the
term 'qualia', both in Kirk's paper and throughout
this paper, just stands for the properties of subjects that make
them, in the constitutive rather than causal sense, qualitatively
conscious. Qualia are simply the properties we have and zombies lack
just in virtue of our being conscious. The acquisition of qualia
seems quite straightforwardly to transform the zombie from a
non-conscious to a conscious being, and in fact into a being just
like us. Kirk's argument then proceeds:

4. But my zombie twin is by definition
unaffected by anything non-physical.

5. So he is unaffected by acquiring
non-physical qualia.

6. In order to become conscious as a
result of his acquisition of non-physical qualia
he would have to be in some way affected by them

7. Since he is not so affected, he,
at any rate, does not become conscious. (1999, p. 6).

Of course, in this part of the argument
premise 5 is highly problematic, if not incoherent. It is true that
the zombie is not physically affected by his sudden
acquisition of qualitative mental states, but it does not follow from
this that he is unaffected tout court. To suppose that all
affection is physical affection is just to assert physicalism9.
That is, to a first approximation we can define 'being affected
(by x)' as the acquiring or the losing of a property (because
of x). The zombie, by hypothesis, gains qualia, that is, certain
properties of qualitative consciousness. Thus he is affected. But,
also by the hypothesis of Kirk's reductio, these
properties are radically non-physical. So to deny the non-physical
affection of the zombie is to deny that there are any such properties
to gain, i.e. to assert a physicalism at least as strong as (P). We
already know that if physicalism is true there can be no zombies so
we have here no argument against the existence of zombies independent
of the assertion that physicalism is true. No friend of zombies will
deny that physicalism implies that zombies are impossible, but
equally no friend of zombies will be impressed by an argument from
physicalism to the impossibility of zombies.

But Kirk is not offering such a
simple-minded and hopeless argument against zombies however. Points 4
through 7 are just the warmup. He wishes to deduce from the fact that
the zombie will not be physically affected by the acquisition
of qualia the conclusion that the zombie will not notice or
think about his transformation or the qualia that produce that
transformation. From this he attempts to deduce further that if
zombies are possible then we too, being in effect zombies with
qualia, as in the thought experiment, cannot notice or think about
our qualia, or even tell the difference between different qualitative
experiences.

Let us first consider the claim that the
zombie cannot notice the qualia that it acquires. Here the
assumption, dormant until now, that the physical laws governing the
zombie (or the zombie world) are the same as the actual physical
laws, comes into play. It has to be further assumed that these laws
guarantee the causal closure of the physical world. As noted
above, this is the assumption that every physical event is determined
entirely by physical conditions and events to the extent that it is
determined at all. We don't know for a fact that the causal
closure of the actual physical world is true, but very fundamental
conservation laws in physics seem to suggest that it must be if our
physics is at all on the right track10.
I'm happy to grant physical causal closure for the sake of the
argument. Given this assumption, it indeed must be the case that the
acquisition of non-physical qualia cannot have any indirect or
downstream physical effect on our zombie any more than it can
have an immediate physical effect.

Kirk's argument then proceeds as
follows:

8. Telling the difference between the
subjective character of two perceptual experiences requires detecting
them; and that requires ... being sensitive to or affected by them.

11. Since the friends of zombies
maintain that I am nothing but a compound consisting of a 'zombie
companion' and non-physical qualia ... their position entails
that nothing can detect differences between non-physical qualia,
hence nothing can tell the difference between the subjective
character of smelling tea, and that of smelling coffee.

12. But many of us can tell the
difference between the smell of tea and the smell of coffee.

13. Therefore we are not compounds of
the sort the friends of zombies maintain we are, and zombies in their
sense are not genuinely [logically] possible. (p. 9)

This argument compresses several
intermediate inferences. First there is the claim that if zombies are
logically possible then we (ordinary conscious human beings) are
compounds of a purely physical zombie plus non-physical qualia.
Although rather a perverse way to put the point it must be correct. I
say 'perverse' since on the assumption of the possibility
of zombies the only possible difference between us and them is
the non-physical qualia that we possess and so of course we must be
'physical zombies' plus qualia. Or, to put it more
sensibly, if we think of ourselves minus qualia we end up with
zombies and if zombies are logically possible then it is coherent to
think in this way.

Now, I (and Kirk) take it that being
able to tell the difference between, for example, the taste of tea
and that of coffee involves more than mere behavioral discriminative
abilities, for the zombies possess these but are nonetheless said to
be unable to 'tell the difference' between the taste of
tea and coffee. What they lack, then, is no physically manifestable
ability but rather the kind of knowledge of what tea, or
coffee, taste like which conscious experience affords (readers will
recognise that we are very close to Jackson's famous Mary
argument here, see Jackson 19??). Normally, we think that it is this
knowledge that grounds the ability to discriminate the taste of tea
from coffee but that is true, at best, only in normal cases of human
perception. We could, in principle, devise a machine that could
chemically discriminate coffee from tea with as much accuracy, and
similar categorizing dispositions, as human beings, but we don't
think that such machines thereby necessarily possess any qualitative
consciousness of tastes. Non-conscious discriminators are certainly
possible (in fact there seems to be a great many of them, from
thermostats to sunflowers). Zombies are an extreme case of
non-conscious discriminators, which possess exactly the same
discriminatory powers that we do, but without the consciousness that
normally attends such powers.

Thus, of course, to all appearances,
zombies are able to tell the difference between the taste of
tea and coffee. My zombie, for example, will utter sounds which can
be interpreted as English statements about how different the taste of
tea is from that of coffee, and in what ways these tastes differ. Do
these utterances have 'real meaning'? I don't think
the answer to this question is at all obvious, but let us suppose
that zombies can speak real English (or a homonymous and synonymous
variant of English) so that when they claim, for example, that they
are conscious they are making a genuine claim, a claim which is,
unfortunately for them, simply false.

If they can make such claims, then
presumably zombies can have real beliefs. Again, it is not really
obvious to me that this is the correct interpretation of zombie
behavior and neural activity11.
But let's suppose that this is OK, so
that zombies do have genuine beliefs about various things; most
significantly they have beliefs about qualitative states of
consciousness. One might wonder, I suppose, how zombies could acquire
concepts of states of qualitative consciousness in the complete
absence of consciousness, even if it is granted that they can possess
concepts in general. But blind people can have concepts of visual
consciousness despite lacking it, so it is difficult to claim there
is a difficulty of principle here. Many of the zombies' beliefs
about qualia are true beliefs, such as the belief that the smell of
roses is a pleasant smell, or the belief that there is a certain
thrilling feeling that attends riding a roller-coaster. But all their
substantive beliefs about their own conscious experiences are
uniformly false (althought they have, as do we, various logically
trivial beliefs 'about' self-referred experiences which
are true by default, such as the belief that right now I am either
having the experience of tasting cinammon or I am not). Of course,
zombie beliefs are one and all non-conscious beliefs,
whereas many of our beliefs are conscious. But there doesn't
seem to be anything incoherent in the idea of non-conscious belief.12

These zombies will even have, or appear
to have, ostensive beliefs of the form: so
strawberries taste like that (just after 'tasting'
their first strawberry). The ostensive act here will fail simply
because of a lack of a referent for the demonstrative. However, there
is nothing particularly mysterious in themselves about such beliefs,
if they are beliefs, nor anything about them which especially
concerns the zombie problem. We have all at one time or another had
this sort of false belief, or 'cognitive failure'. It is
of course very weird that zombies are so systematically mistaken even
about such intimate judgements about their own (putative) experience.
But no one claims that zombies aren't weird; lots of logically
possible things are weird (like flying pigs, the worlds of Harry
Potter or Frodo Baggins, etc.). The question is whether zombies are
as weird as square circles.

Notice that if it was logically
impossible for something to have beliefs about things that could not
physically affect it, then zombies could have no beliefs about these
presumed non-physical properties of qualitative consciousness. Then
it might seem there would be a quick route towards the conclusion
that zombies are logically impossible. The argument would go like
this. Zombies could not have any beliefs about qualia, either before
or after their transformation from qualia-less to qualia-enjoying
creatures. But then they could not tell the difference between
distinct qualia just because 'telling the difference'
involves, at least, having some beliefs about qualia. But we are, or
are equivalent to, zombies that have acquired qualia. So therefore we
can't tell the difference between distinct qualia. But since we
can, we get a reductio of the claim that zombies are logically
possible.

This is not a very convincing argument
however, because it rests on the extremely dubious premise that it is
impossible to have beliefs about what cannot physically affect one.
Beliefs about mathematical objects or abstract entities in general
present obvious difficulties here, as do beliefs about possible
physical things that we cannot be in physical contact with (e.g.
events outside our 'light cones'). In general, the
difficulty with this line of argument is to present a case for this
restriction on belief, and concept, formation which is independent of
the assumption of physicalism, an assumption which would in this
context once again beg the question. In any event, I don't want
to explore this avenue further here, since it plays no part in Kirk's
argument.

Having granted that zombies can have
genuine beliefs, let us see how the rest of Kirk's argument
fares. I think it rests on a basic mistake in epistemology. It is a
mistake to think that there has to be any physical change in a
believer to transform beliefs into knowledge. I think that however
knowledge is to analysed, it will be possible for all the conditions
of the analysis to be met save for that demanding the truth of what
is believed. This is obviously true for common theories of knowledge.
On justified true belief accounts, it is
easy to imagine situations where the only thing preventing a belief
from attaining the status of knowledge is the fact that the belief is
false. There are lots of justified false beliefs which, if they had
been true, would have amounted to knowledge. Suppose you tell me you
were in Simcoe Hall yesterday afternoon. You are generally
trustworthy and I understand your utterance, and thus form the belief
that you were in Simcoe Hall yesterday. Because of a slip of the
tongue however, you mispoke yourself; you were actually in Simcoe
Hall the day before yesterday. So obviously I don't know
that you were in Simcoe Hall yesterday, but I would have known if you
had been (everything else in the siutation being kept the same). A
similar point clearly holds for reliability theories of knowledge
and, I would venture to assert, any other reasonable account.

I think zombies are in an epistemic
situation akin to my example, though one as all encompassing and
strange as befits their peculiar nature. They have every reason, so
to speak, to believe that they have qualia (always recalling that we
have granted that they can have and, in virtue of their understanding
of English and physical indistinguishability from normal human
beings, do have beliefs about qualia). They are, to use another
model, epistemically virtuous, at least they are as virtuous as we
are, and if we can manage to have knowledge about qualia then so
could they -- if they only had any qualia to have
knowledge about.

So what happens when, as Kirk imagines,
a zombie suddenly acquires consciousness? First, the zombie is
affected, but non-physically, just by gaining certain properties that
it lacked before. Of course, nothing physical changes, by hypothesis.
And yet the zombie's false beliefs about its own mental states
suddenly become true, and they become knowledge too. Friends of
zombies will say that the zombie now knows what things taste like in
virtue of actually having the conscious experiences which carry this
information, whereas the earlier, non-conscious beliefs about tastes,
referred to nothing, hence were false, hence could not
constitute knowledge about how things taste. Zombies are, so to
speak, very close to being able to tell the difference between
tea and coffee, lacking only one, alas essential, component; they are
like the man who says 'I'd be rich if I only had money'.

Consider this from the side of the
zombie. Suppose a zombie is asked to think about or 'internally
compare' the tastes of coffee and tea. The zombie thinks for a
while, and carefully sips some tea and coffee, then
soliloquizes about this 'difference' for awhile,
doubtless saying many things that are true of the tastes of tea and
coffee. But the zombie's remarks are grounded in utterly false
beliefs simply because the requisite experiences are just missing. Of
course, this kind of zombie would never seriously entertain the idea
that it is a totally non-conscious being. It's epistemic
situation is the same as mine and if I have no good reason to wonder
whether I am a zombie then neither does it. But that doesn't
eliminate the possibility that it is a zombie, any more
than the fact that I have no good reason to believe that I am a brain
in a vat shows that it is logically impossible that I am a brain in a
vat.

One might wonder whether a zombie could
know it was a zombie. I don't see anything absolutely
preventing a zombie coming to believe correctly that it was a zombie,
but I doubt that would be knowledge, since it would be an irrational
belief13.
What evidence could there be in favour of the zombie hypothesis?
Since the zombie hypothesis requires the idea that the zombie
inhabits a very peculiar sort of possible world, I don't think
there could be a route towards a rational belief that it was a
zombie. What could ever lead it to rationally conclude that it was in
such a bizarre, merely logically possible, world?

But doesn't this whole analysis
imply that it is logically possible that I am a zombie, right now,
even as I, apparently, consciously think about my own consciousness?
Not at all -- at most it shows that from your epistemic
point of view it is logically possible that I am a zombie (and if
zombies are logically possible at all this is hardly surprising --
the problem of other minds is a skeptical problem which is not
incoherent). For I have conscious beliefs about my
experience, and they are what form the basis of my knowledge that I
am conscious. If you are wondering whether you are a zombie, I ask
you just to consider whether you are conscious. You have
conscious experiences; you are conscious that you are not a zombie,
and that entails that you are indeed not a zombie. A conscious belief
that one is conscious is self-validating; it cannot be a false
belief14.

It is instructive here to compare my
knowledge that I am not a zombie with my knowledge that the world is
ancient, and was not created five minutes ago in the exact state it
actually was in five minutes ago. Although I do know that the world
is very old, it is logically possible, and perhaps more, that I am
wrong. The fact that knowledge that P logically implies that P is
true does not threaten the claim that what is known might be false.
It is an analytic fact about knowledge that:

NEC(Kp ==> p),

but of course it does not follow that

Kp ==> NEC(p).

My epistemic situation with regard to
the age of the world is in fact somewhat more perilous than this
suggests. The fact that I consciously believe that the world is
ancient -- which I do of course -- is logically independent
of the age of the world, that is

POSS(CBEL(Ancient) &
~Ancient ).

Contrast this with the status of my
knowledge that I am not a zombie. There, I am on much firmer ground,
for my conscious belief that I am not a zombie logically implies that
I am not a zombie, that is,

NEC(CBEL(~zombie) ==> ~zombie).

It
is logically impossible that I, in my current state of consciousness,
am really only a zombie. Of course, if zombies are logically possible
then there is a possible being who is physically just like me but who
is a philosophical zombie. But that being can't be me, simply
because I consciously believe that I am not a zombie while my zombie
duplicate does not. It's also true that, so far as I can tell
'from the inside' it is logically possible that I might
become a zombie in the next instant (I might lose consciousness for
all sorts of reasons). But if this did happen I would then lack the
states of consciousness which currently ground my knowledge that I am
not a zombie; right now, I
have such states, I am
aware of them, and they logically preclude my being a zombie (a power
which the consciousness of my belief that the world is ancient
utterly lacks about its object).

So if I'm not a zombie, I know I'm
not a zombie. And, of course, the converse holds as well, so: I'm
not a zombie iff I know I'm not a zombie. The zombie has no
such beliefs, even though it unconsciously -- or, better,
non-consciously -- believes that it is conscious. Too bad for
it, all such beliefs are false.

Kirk's
argument against the logical possibility of zombies does not succeed.
I think I've given a logically consistent tale of how zombies
could transform into conscious beings such that this transformation
would give them knowledge of the difference between the experiential
qualities enjoyed in consciousness. This does not show that zombies
are logically
possible, but it does support that contention, for the burden of
proof must always lie on those seeking to deny that a proposition is
logically possible, since the claim of mere logical possibility is
inherently a very weak claim. Physicalists should remember that their
doctrine entails that zombies are logically impossible, which may
give them some comfort. On the other hand, any argument based upon
this entailment advanced in support of physicalism would be viciously
circular. Thus, any argument against the logical possibility of
zombies which aims to bolster physicalism has to be independent of
the claim of physicalism. I think it will be hard to come up with any
such argument; and I don't think that Kirk has done so.

Finally, let us consider the worry of
epiphenomenalism, which may seem pressing
at this point. That is, it may seem that if zombies are logically
possible then qualia have got to be epiphenomenal, since a
philosophical zombie can be transformed into one of us without any
change in the physical world. Now, it may be that a philosophical
zombie which is miraculously endowed with consciousness, as in Kirk's
thought experiment, will have only an epiphenomenal consciousness.
It's important to see that this does not entail that our
consciousness is epiphenomenal. First, let's get clear about
the kind of epiphenomenalism that is at issue here. I will call it
'physical epiphenomenalism', meaning that consciousness
has no physical effects. Perhaps in theat distant world in
which some erstwhile zombie suddenly acquires consciousness, these
mental properties obey their own intra-mental causal laws, and have
their own causes and effects within the mental domain, But it seems
that, on the assumption of physical closure, nothing physical can
change because of the introduction of consciousness and thus the new
mental properties cannot have any physical effects and thus they
count as 'physically epiphenomenal'.

The thought experiment of the zombie who
acquires qualia thus perhaps shows that epiphenomenalism is itself
logically possible (as I am inclined to suspect it is in any case).
But it does not follow that our consciousness, in this
world, is physically epiphenomenal. This can be demonstrated very
simply: overdetermination is not logically
impossible. Given Kirk's implicit assumption of the causal
closure of the physical world, there is a purely physical sufficient
cause of every physical event. Given that the logical possibility of
zombies shows that the properties of consciousness are radically
non-physical, the efficacy of these properties cannot ride directly
on the efficacy of physical properties (as they do in, for example,
identity theories). It does not follow that qualia are epiphenomenal
since they may be overdetermining causes of physical effects. The
laws of nature in the actual world include laws linking physical
states to states of consciousness and, it may be, laws linking states
of consciousness to certain physical states. In the logically
possible world where the zombies live, these laws are broken (by
hypothesis), but the physical laws are left unchanged. It may be that
the result is that any qualia introduced into that world become
physically epiphenomenal. But our world is not like that, or at least
not necessarily like that. The mere possibility of overdetermination
shows that the possibility of zombies does not entail
epiphenomenalism. The fact that qualia are epiphenomenal in the
zombie world does not imply that they are epiphenomenal everywhere.
Fundamentally, this is because causation is not a matter of logical
necessity. The fact that it is logically possible that a massive
object released near the Earth will not fall does not show that
gravity lacks causal efficacy.

So the logical possibility of zombies
does not imply epiphenomenalism. Would the logical possibility of
epiphenomenalism imply the logical possibility of zombies? Not
directly. It is not clear how to argue that the existence of a
possible world in which qualia are physically epiphenomenal
entails that there is a zombie world, a world where qualia are
lacking altogether despite that world's total physical
similarity to the actual world. But perhaps the possibility of
epiphenomenalism strengthens the case for zombies insofar as it
easier to suppose that inefficacious
properties can be eliminated from a world, thus transforming an
epiphenomenalistic world into a zombie world.

However, the logical possibility of
epiphenomenalism does share something important with the logical
possibility of zombies. They both refute physicalism. This follows
from the assumption that physical properties, in a world that shares
our physical laws, are efficacious (because efficacy is guaranteed by
the laws, so that if we have the same laws we'll have the same
cause-effect relations). If there is such a world where qualia are
inefficacious, then evidently qualia cannot be physical properties.
And, on the twin assumptions that functional properties supervene
upon the physical and that functional properties are efficacious, the
possibility of epiphenomenalism also shows that qualia cannot be
functional properties.

Some possibilities that are mere logical
possibilities would thus have some very significant consequences for
physicalism. Physicalists ought to be worried about them, and seek
some arguments independent of their physicalism to show that these
are only apparent logical possibilities. I have argued that Kirk's
efforts do not succeed, but perhaps there are other ways to show that
zombies are no more possible than square circles, or colourless green
ideas.

William Seager

University of Toronto at Scarborough

1The
physical realm is causally closed just in case every physical event
is determined to occur and have the properties it has only by other
physical events (to the extent that it is determined to occur at
all). Thus if the physical realm is causally closed then two worlds
that are indistinguishable physically up to, or perhaps simply at,
some point in time will evolve in physically indistinguishable ways
thereafter, modulo any possible indeterminacy inherent in physical
law. Indeterminacy would be accommodated by looking at the class of
possible futures licensed by indeterministic physical law: two
worlds identical up to, or perhaps simply at,time t will share exactly the same class of possible futures or,
in other words, the same futures will be ruled out as inaccessible
from these worlds as they are at time t.

2Many
of these tests are quite independent of me. For example, if my
reasons for believing in an ancient world have, unknown to me, been
undermined then perhaps my belief will not count as knowledge (the
Gettier examples are simple cases of undermined evidence, although
very much more subtle cases are possible). Here we see some of the
subtlties that abound in epistemology. Of course, my belief that
there is no undermining evidence unknown to me bearing upon the age
of the world is also on a pretty sound epistemological footing.
Sound enough, in fact, to underwrite my knowledge claims about the
lack of such evidence, and hence the age of the world.

3The
caveat is needed since physicalists
disagree about whether or not physical history and physical
environment must also be duplicated to avoid zombiehood. Fred
Dretske's theory of consciousness, for example, actually
predicts zombies which are
locally physically
identical to normal human beings, but must differ in their history
of physical interaction with the world (see Dretske 1995). See below
for why the caveat makes very little difference to the zombie
argument against physicalism.

4So
far as this argument goes there could still be creatures very much
like us who lacked consciousness in a world that was perhaps only
subtly different from the actual world. If this is a genuine logical
possibility then it too is a problem for physicalism, but not one to
be discussed here (see Kim ?? on global supervenience).

5I
am assuming that a world in which certain 'laws' held
but were violated by miracles
is a world with different
laws from those of the actual world. Or, to put it another way, I'm
assuming that the actual world is subject to a kind of 'metalaw':
there are no miracles.

6Descartes
himself would have denied the possibility of philosophical zombies,
because a being physically identical to one with a mind would not be
able to behave in all ways like the being with the mind (for
example, the putative zombie would not be able to speak sensibly in
general, do mathematics, etc.). According to Descartes,
matter-in-motion just does not have the intrinsic resources to
organize fully or creatively intelligent behaviour. However, even on
a Cartesian account, there could be 'temporary' zombies.
This is evident because animals are
zombies according to Descartes even though they engage in very
complex behaviour susceptible to mentalistic explanation. According
to Descartes, humans, too, seldom use their minds to guide their
behaviour; most of the time the body is left to its own devices.
Kirk makes what is a perhaps common error about Cartesianism
where he says that on the Cartesian doctrine, 'an exact
physical replica of myself [who lacked its
Cartesian mind] would not behave like me; it would collapse
like a disconnected marionette' (1999, p. 2). On the contrary,
such a being would be something akin to an ape, able to get on quite
well, but incapable of scaling the heights of distinctively human
intelligent behaviour. Thus there could be fairly long stretches of
behaviour during which the merely physical, and utterly
non-conscious, replica would
be behaviourally indistinguishable from
the original.

7Note
the extreme case of things that are totally non-physical, i.e. which
have no physical properties. Any two such things are physically
identical and thus any property which the one can have and which the
other can lack will count as a radically non-physical property. This
is as it should be. Logically trivial properties, such as either
possessing an essential non-physical component or not, which
everything must have, do not count as radically non-physical. That
will not cause any troubling consequences in what follows. Nor does
it seem exactly wrong to classify them as such, since every physical
object will also have such properties (trivially).

8Note
that by the phrase 'zombie world' I am not implying that
this is a world devoid of consciousness, only that we may require
physical identity of a whole world to generate even a single genuine
philosophical zombie.

9Perhaps
Kirk intends to define zombies so that they cannot be
affected in any way whatsoever by non-physical properties. If so,
zombies cannot acquire qualia and Kirk's argument becomes
incoherent (a point made by Michael Neumann in personal
correspondence).

10Very
complex issues lurk here. The status of energy conservation in
general relativity, for example, is very vexed and rather unclear.
We also know that quantum mechanics permits 'temporary'
violation of energy conservation via the time-energy uncertainty
principle. Could the mind exploit such indeterminacy? Orthodoxy
would doubt it.

11It
seems defensible to claim instead that zombies are completely
mindless; that in the absence of consciousness there are no mental
states at all (this seems to be Searle's (19??) position based
on the claim that mental states are those states that have the
potential to become conscious to their subject). However, even
taking such a strong line would still leave us with the zombies'
mentalistically interpretable behaviour
(both outer and inner, neurological). Thus we could define states of
F-belief ('F' for faux),
F-desire, etc. which zombies possess and which are, from the
outside, so to speak, semantically evaluable
and explanatory of behaviour. Whether we pretend that the 'F-'
is there or not seems to make little difference in the end, at least
in the matter of unconscious mental states.

12Zombies
can also have higher order beliefs about their own mental states (or
F-mental states). Thus if zombies are logically possible then higher
order thought (HOT) theories of consciousness (see Rosenthal 1986)
are refuted. So, for that matter, are the first order
representational (FOR) theories that make consciousness a matter of
the impact, or potential impact, of possibly non-conscious mental
states on the belief-desire system (see Dretske 1995 or Tye 1995).
Zombie representational states have such impacts without generating
consciousness. Or, to put it another way, if HOT or FOR theories are
true it is logically necessary that they
be true. If there is a logically possible world which refutes a HOT
theory it will be a world with zombies (or, somewhat more
cautiously, a world with 'partial' zombies --
creatures with the requisite HOTs to make state X conscious while X
remains non-conscious).

13I
do not believe there is any psychiatric or neurological syndrome in
which patients claim to be philosophical zombies. In Cotard's
delusion, people claim to be dead, but they are only claiming to be
'Hollywood zombies'. They have not and do not think they
have lost consciousness, as is evident in their claims that they can
smell their own flesh rotting or feel worms crawling over them.

14Thus
I am, in a way, disagreeing with another of Kirk's premises,
namely that 'qualia cannot detect qualia' (1999, p. 8).
Kirk's reason for accepting this premise is that it is
trivially true because qualia are 'properties, not acts, much
less acts of comparing or distinguishing' (1999, p. 8). While
that is true, there might be other ways, as suggested in my text,
that having qualia
validates, or transforms into knowledge, certain beliefs about
qualia. Kirk perhaps also neglects the fact that there is conscious
thinking as well as conscious experiencing.